Combustion misfires lead to an increase of the toxic substances, which are emitted during the operation of the internal combustion engine, and can lead to damage of a catalytic converter in the exhaust-gas system of the engine. To satisfy statutory requirements for on-board monitoring of exhaust-gas relevant functions, a detection of combustion misfires is necessary in the entire rpm and load ranges. In this connection, it is known that characteristic changes of the rpm characteristic trace or curve of the internal combustion engine occur during operation with combustion misfires compared to the normal operation without misfires. One can distinguish between normal operation without misfires and operation with misfires from the comparison of these rpm traces.
A method, which operates on this basis, is already known from U.S. Pat. No. 5,955,663.
In this known method, a crankshaft angle region, which is identified as a segment, is assigned to a specific region of the piston movement of each cylinder. The segments are realized, for example, by markings on a transducer wheel coupled to the crankshaft. The segment time is the time in which the crankshaft passes through this angular region and is dependent, inter alia, on the energy converted in the combustion stroke. Misfires lead to an increase of the ignition-synchronously detected segment times. According to the known method, an index for the rough running of the engine is computed from the differences of segment times. Additionally, slow dynamic operations, for example, the increase of the engine rpm during a vehicle acceleration, are compensated by computation. A rough-running value, which is computed in this way for each ignition, is likewise compared ignition-synchronously to a predetermined threshold value. This threshold value is dependent, if required, on operating parameters such as load and rpm and exceeding this threshold value is evaluated as a misfire.
The reliability of the method is decisively dependent upon the accuracy with which the rpm differences of the crankshaft, which are characteristic for misfires, can be determined from the segment times.